In the Sun the primary process is called the "proton-proton" chain.
In this reaction four protons (these are the hydrogen nuclei) are
fused into one helium nucleus (also called an "alpha particle").
An alpha particle has about 1% less mass than four protons. The
rest of the mass is turned into gamma rays - high energy photons.
The Sun converts about 5 million tons of mass into energy every
second. (Don't worry, there's enough for a long time!)

Here is the reaction in chemical notation

4 1H + 2 e --> 4He + 2 neutrinos + 6 photons

Fusion can only happen when they can get close to each other. It is a
process that actually has several stages because the protons ("1H")
both have a positive charge and at lower temperatures they repel each
other. But because the gas in the center of the Sun is so hot, the
protons can hit each other with high speed. When they get close to
each other a natural force in nature for tiny distances, the strong
nuclear force, can bind the particles together. Several other
intermediate steps take place where electrons and antielectrons
annihilate each other, more protons collide into each other and a
helium atom is made and with the helium and neutrons and protons
bouncing around in the hot gas, when they get close to each other, new
particles are created.

So you see our one equation above has many steps! In chemical
notation it looks like: